This application claims the priority of German Application Nos. 100 49 176.6, filed Oct. 5, 2000 and 101 39 686.4, filed Aug. 11, 2001 and International Application No. PCT/EP01/11360, filed Oct. 2, 2001, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a piezoelectric expansion actuator.
The use of d31 piezoplates or d31 piezosegments is known for the purpose of vibration control and to influence vibrations in structures. d31 piezoplates take advantage of the elastic transverse contraction of the piezoelectric material. Several piezoplates or piezosegments will be described as piezoelectric stacks. A piezoelectric stack consists of several, but at least 2 piezoelements. With the above d31 piezoelements, for example, expansions are introduced into carrier structures for helicopter transmissions so as to suppress the transmission of body sound onto the helicopter cell. In doing so, the d31 piezoelements are integrated in accordance with their expansion direction, which acts parallel to the surface of the d31 elements, into the surface of the carrier structure across a large surface, e.g. through an adhesion technique.
By contrast, the expansion in the familiar d33 piezoelements acts perpendicular to the surface of the elements because d33 piezoplates take advantage of the expansion of the piezoelectric material in the direction of the applied field.
German Patent No. 198 13 959 A1 describes a device for body sound suppression that more effectively reduces the transmission of equipment vibrations and oscillations through a carrier structure onto a cellular structure of a cockpit in a simple construction and at relatively low integration complexity. German Patent 198 13 959 A1 provides that the sound suppression device includes at least one piezoactuator, which introduces the oscillations into the carrier structure in order to block the body sound transmission path onto the insulating structure substantially and to compensate acoustic excitation by use of the existing and excited system dimensions of the sound generator more effectively. This technical idea is not limited to use in helicopter manufacturing. It can be employed in all areas of mechanical engineering where a device for body sound suppression becomes necessary.
Contrary to other familiar expansion actuators, the piezoactuator does not implement the application of power onto the carrier structure at only points, but rather across a relatively large surface of the carrier structure. The carrier structure can be arranged for example between the main gearbox of a rotor and a cellular structure of the cockpit of a helicopter. In this case, the carrier structure would be one or more struts (also called gear struts). The piezoactuator is largely arranged along the entire circumference of the strut and exhibits a defined expansion in the axial direction of the strut. Forces are introduced by the piezoactuator pursuant to German Patent DE 198 13 959 A1 via its surface.
The efficiency of power application is limited by the effective surface of the strut that is to be covered.
The invention is based on the development of a piezoelectric expansion actuator for d33 piezoelements, with which vibrations can be suppressed in structures, and furthermore of considerably increasing the efficiency of power application of a piezoactuator despite the contrary tendency of decreasing construction volume of the piezoactuator.
A solution pursuant to the invention is based on the fact that a d33 piezoelement in the form of a stack is clamped into a mechanical frame, which is fastened to the surface of the structure. Apart from a highly specific, mechanical power, the expansion actuator also achieves good efficiency. Also beneficial is the application of mechanical pre-stress that is integrated in the actuator which allows critical tension strain to be avoided for the piezoelements. Optionally, devices can be integrated in the frame with which stroke speed transformations or stiffness transformations can be beneficially achieved.
In another solution pursuant to the invention, the efficiency of power application for the piezo actuator can be improved by considerably increasing the distance between the resting areas of two output elements of a piezoactuator and a corresponding end plate of the piezoelectric stack in the axial direction towards the strut end. The output elements of the mechanical frame form the power transmission device from the piezoactuator to the strut. The thus considerably enlarged strut distance between the resting areas of the two output elements exhibits less stiffness, consequently leading to an expansion of this strut section with less force required than in a comparable configuration of a piezoactuator where the distance of the resting surfaces of the output elements largely corresponds to the length of the piezoelectric stack. The piezoelement also uses the d33 piezoelectric elements.